This invention relates to electrodes for use in electrolytic processes and in particular for use as an anode in processes involving the electrolysis of aqueous solutions of alkali metal halides.
A variety of materials have been tested and used as chlorine anodes in electrolytic cells. In the past, the material most commonly used for this purpose has been graphite. However the problem associated with the use of graphite are several. The chlorine overvoltage of graphite is relatively high, in comparison for example with the platinum group metals. Furthermore, in the corrosive media of an electrolytic cell, graphite wears readily, resulting in substantial loss of graphite and the ultimate expense of replacement as well as continued maintenance problems resulting from the need for frequent adjustment of spacing between the anode and cathode as the graphite wears away. More recently a variety of dimensionally stable anodes comprising a coating of a platinum group metal oxide on a conductive substrate have been developed and used in chlor-alkali cells. Anodes of this type provide excellent electrocatalytic properties as well as a considerably lower wear rate then graphite and are used extensively for the electrolysis of brines. However, the cost of platinum group metal oxides is relatively high and, although the wear rate of such materials is relatively low compared with that of graphite, some wearing away does occur during normal cell operation with attendant loss of the expensive platinum group metal oxide. Efforts to minimize the amount of platinum group metal oxide employed in such anodes have resulted in the development of various anode coating compositions comprising a mixture of a platinum group metal oxide and a non-platinum group metal material. Among these are anode coating compositions comprising a platinum group metal oxide in admixture with one or more other metal oxides such as an oxide of tin, titanium, tantalum or other. Anode coating compositions of this type require lesser amounts of the expensive platinum group metal-oxide while retaining at least some of the excellent electrocatalytic properties or other advantages of these materials. Among the important properties to be considered in such anode coating compositions are the compatability of the materials, the stability of the composition in the anode environment of an electrolytic cell, and the electrocatalytic characteristics of the composition, as compared with the platinum group metal oxide alone.
Therefore it is an object of this invention to provide improved electrodes, especially useful as anodes in electrolytic processes. It is a further object to provide platinum group metal oxide anodes wherein the amount of platinum group metal oxide required is substantially reduced without an attendant diminution of electrocatalytic and other advantageous characteristics. It is a still further object to provide such anodes comprising a compatible combination of oxides of a platinum group metal and a non-platinum group metal. It is a further object to provide a novel method and electrolytic cell apparatus for the electrolysis of brines. These and other objects and advantages of this invention will become apparent from the following description.